50hp 1986 Yamaha

[Northriverbob52]

The bottom cylinder is not firing, switched coils back and forth and still not firing..Spark plug is clean...It's getting gas and has compression...Someone said it's probably a crankcase seal leaking and not allowing the cylinder to fully pressure up????.
Could it be the electronic module that controls the coils and spark
Thanks
Bob

[jb123]

not firing ...as in SPARK??
you check and swap coils so we assume you mean no "spark"
what does c/c compression have to do with no spark, obviously its needed
comp check ?
usually its a dirty carb

[99yam40]

Bottom cylinder will not fire properly even with good spark and compression if the fuel pump is leaking fuel into the crankcase due to pin hole in diaphragm.
When was the last time you changed it? or even checked it?

[Northriverbob52]

Bottom cylinder will not fire properly even with good spark and compression if the fuel pump is leaking fuel into the crankcase due to pin hole in diaphragm.
When was the last time you changed it? or even checked it?

I checked the fuel pump and took it apart...the diaphram looked ok but was not looking for a pin hole

A mechanic in Portland, Or said it was a crank seal but still don't know how that can effect the lower cylinder....

[99yam40]

If it is sucking air through the bottom seal then it will make the cylinder fuel/air mix to lean to fire properly

Maybe spray some fuel mix into the bottom carb throat to see if it will cause it to start firing while running

[Northriverbob52]

Will try this after the holiday weekend...going to change the fuel pump diaphram first and the try spaying mixure into lower carb...Thanks for your help